Altruism needs selfish genes to evolve after all

It’s a problem that has been debated ever since Darwin&colon; how have hundreds of species of insects and other animals evolved altruistic helpers that give up their own reproduction for the sake of others?

Recently, the orthodox explanation – that they favour their own genes indirectly by helping their kin – has been fiercely challenged by Edward O Wilson, one of the most prominent evolutionary biologists of our time.

But now a research team led by William Hughes, of Leeds University, UK, claims to have falsified Wilson’s predictions, showing that genetic relatedness is really the key.

At the core of the dispute is the theory of kin selection, formalised in the 1960s by William Hamilton, accepted by the vast majority of modern biologists and defended by Richard Dawkins. According to Hamilton’s rule, apparent acts of altruism – foregoing reproduction to help others, say – are actually self-serving, because they benefit the altruist’s genes.

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Wilson broke with this view, proposing that altruism evolved because it benefits groups, rather than genes. For such “group selection” to take place, he argued, animals don’t need to be closely related, they only need to stick together and cooperate.

Multiple males

He argues that this is more likely to occur when individuals tend to remain in the nest they are born from. So the high relatedness observed in ants, bees and wasps – so-called eusocial species that have a queen and sterile workers – is a consequence, not a cause, of altruism.

If Wilson is right, then there should be no correlation between the degree of genetic relatedness within insect colonies and the level of social cooperation they show.

To test this, Hughes and colleagues looked at a behaviour that has fundamental consequences for colony kin structure – polyandry, which occurs when females mate with more than one male. This enhances female fitness by producing more variable offspring and is a common behaviour throughout the animal kingdom.

“Birds, reptiles, flies, butterflies, beetles – pretty much all species that have been looked at show some level of polyandry,” says Hughes.

Ancestral monandry

Hughes and colleagues looked at the levels of polyandry in 267 species of eusocial ants, bees and wasps. The last common ancestor of these insects was solitary, and eusociality evolved independently on eight different occasions. By looking at how the species are related to each other over evolutionary time, the team could reconstruct the ancestral condition – monandry or polyandry – in each case.

The team found the ancestral condition was invariably monandry. And the same applies to termites, shrimps, ambrosia beetles and most other eusocial organisms.

“[In species excluding ants, bees and wasps] the data is much more limited, but it points in the same direction,” says Hughes. “You always have ancestral monandry when eusociality evolves.” In other words, close genetic relatedness is crucial to the evolution of altruism.

Eventually, once eusociality is evolved and established, insect queens start reaping the benefits of multiple mating, which has evolved several times as a secondary condition, says Hughes.

‘Cut and dry’

Intriguingly, very high levels of polyandry are only observed in species where helpers have entirely lost the ability to reproduce, becoming permanently sterile castes. Again, this is exactly what kin-selection theory predicts, because only when eusociality has become irreversible, and workers have no other option but to help, can the leash of genetic relatedness loosen.

Hughes contents that these results would seem to settle the longstanding debate revived by Wilson.

“Wilson predicted that high relatedness evolves after eusociality. We show that it is ancestral. It’s pretty cut and dry, really,” says Hughes.

Wilson, however, does not agree that the debate has been resolved so cleanly.

“Hughes and colleagues did not prove the correlation of eusociality and ancestral monogamy, because they have no data on the many lines that did not evolve eusociality,” he says.

“And they failed to mention other published explanations of multiple insemination in the later stages of eusociality. The weight of evidence favors the new explanation of close kinship as a consequence of eusociality, as laid out in my BioScience article.”

Hughes accepts that there is much more to altruism than simple genetic benefits.

“It is good to be challenged about our hypotheses,” says Hughes, “Hamiton’s equations have three components in them but we have become very focused on relatedness. [Wilson] has done us a service in drawing attention back to ecological benefits and other components.”